Tuatara

tuatara (genus Sphenodon), Sphenodon punctatusEncyclopædia Britannica, Inc.any of two species of moderately large lizardlike reptiles endemic to New Zealand. Although a growing number of geneticists contend that all living tuatara belong to the same species, two species of extant tuatara are recognized, Sphenodon guntheri and S. punctatus. These two species, and possibly other now-extinct species, inhabited the main islands before the arrival of the Maori people and the kiore—the Polynesian rat (Rattus exulans). S. guntheri lives on a few islets in the western Cook Strait, and S. punctatus inhabits the North Island and approximately 30 islets off the island’s northeast coast. Tuatara is the Maori word for “peaks on the back.”

Tuatara are the sole survivors of an ancient group (order Rhynchocephalia [sometimes called Sphenodontida]) of reptiles that first appeared in the fossil record of the Middle Triassic Period, some 238–240 million years ago. The oldest fossils comparable to Sphenodon, the only extant genus of tuatara, are jaw fragments from South Island dated to 19 million to 16 million years ago; however, most known fossils of the genus are less than 10,000 years old. Tuatara are distantly related to the reptiles of order Squamata (lizards, snakes, and amphisbaenians [worm lizards]).

Natural history

Tuatara are largely, but not exclusively, nocturnal animals. They regularly bask during daylight hours at the mouth of their burrows. However, they become much more active at night, foraging in and around their burrows and interacting with other tuatara. While capable of digging their own burrows, tuatara often use those of the fairy prion (Pachyptila turtur), a burrow-nesting seabird. These settings provide tuatara with protection and food. Excreta from the birds supply a large arthropod community, and the tuatara prey on the diverse array of arthropods and smaller lizards that use this resource. Tuatara may also prey on fairy prion eggs and chicks.

The burrow systems are often made up of a network of interconnected tunnels. Each tunnel may be several metres long and possess multiple entrances. Although tuatara are solitary and territorial, as many as five or six tuatara may occupy the same system. However, they will not share the same tunnel segment, and individuals will often use more than one tunnel system. The social structure of tuatara is similar to that of iguanian lizards (iguanas, anoles, agamas, chameleons, and others) in that territorial defense and courtship behaviours are established through body display and colour changes.

Beginning in January and lasting through March, following the reproductive season of the fairy prion, the mating season for the tuatara occurs. During this period, social interactions between tuatara increase. A male defends his territory by inflating his body, erecting the crest on his head and neck, and shaking his head. Close encounters between males result in a sequence of mouth-gaping behaviours that can lead to chases. These pursuits may culminate in forceful biting if both males are initially unwilling to yield. Males often make croaking sounds during these intense confrontations, but these “vocalizations” are seldom heard at other times and are most likely the by-product of chest compression.

During courtship the male approaches the female, inflates his body, and erects his spines. This body enlargement is accentuated by his stilted, stately walk toward the female and around her. A male may circle a female several times before copulation occurs. If the female remains stationary, the male will approach her, step over her back, and move forward. Since tuatara have rudimentary hemipenes, they cannot truly copulate. To compensate, both participants position their cloacal vents as close as possible to one another, and sperm is transmitted from the male’s cloaca to the cloaca of the female.

For most females, egg deposition occurs on a four-year cycle and not until late October to mid-December. Each female lays 8–15 eggs, which are promptly buried. Each egg requires 11–16 months of incubation in the ground before hatching occurs. The growth of the juveniles is also slow, and individuals require 9–13 years to attain sexual maturity in most populations. Tuatara commonly live 60 years or more, and many individuals are capable of reproduction well into their 20s and 30s; a few can even reproduce after 60 years of age.

The tuatara’s largely nocturnal habits conceal it from many potential predators, but occasionally it is preyed upon by hawks, gulls, or kingfishers. The kiore, or Polynesian rat, preys heavily on eggs and the young, and the tuatara may be regarded as seriously threatened with extinction on islands where rats and other mammals, such as cats and pigs, have been introduced.

Form and function

Physically, S. guntheri and S. punctatus are quite similar to one another. From the snout to the tip of the tail, adults average 50 cm (20 inches) in length and weigh between 0.5 and 1 kg (1 and 2 pounds). Males tend to be larger, often approaching 60 cm (24 inches) in length, and generally weigh twice as much as females. Both species have robust lizardlike bodies with a large head, well-developed limbs, and a stout tail. The snout is distinct because of the presence of fused toothless premaxillary bones that give it a beaklike appearance. The entire body is covered with scales. The body is mottled, and coloration ranges from dirty tan to olive-green to slaty gray. Body colour can change over the animal’s lifetime. Males and females possess a series of broad spines, which are derived from scales, that extend down the midline of the back from the nape onto the tail. The largest spines occur on the neck and trunk.

The eyes are well developed with vertical pupils and a retinal structure adapted for low-intensity light. Tuatara also have a third, or parietal, eye on the top of the head. Although this eye has a rudimentary lens, it is not an organ of vision. It is thought to serve an endocrine function by registering the dark-light cycle for hormone regulation. Tuatara display no ear openings. However, they do have a middle ear cavity with a stapes (a small bone that conducts sound vibrations). They are most sensitive to sounds in the 100–800 Hertz range.

Tuatara are one of the few groups of reptiles that are active at low body temperatures. Their internal temperatures are typically less than 22 °C (72 °F) and usually hover around 18–19 °C (64–66 °F). They can even remain active when their body temperatures dip as low as 13–14 °C (55–57 °F). While they bask in the sun to elevate their body temperatures, life in the burrows and their nocturnal activity expose them to lower temperatures and conditions where thermoregulation is impossible. Through an evening’s activity cycle, their body temperatures will regularly fluctuate across a range of 3–4 °C (5–7 °F). Their reduced activity at low temperatures might also be an adaptation to reduce water loss. To further stem water loss, tuatara excrete uric acid—like their distant reptilian relatives, the squamates.

Evolution and classification

Order Rhynchocephalia is the sister group to the order Squamata, and rhynchocephalids (sphenodontids) share numerous traits with squamates. Both groups possess a transverse cloacal opening (the vent), teeth that are attached superficially to the jawbones, and fracture planes in the tail vertebrae. Rhynchocephalids and squamates also undergo ecdysis, the periodic shedding or molting of the skin to allow growth. Rhynchocephalids differ from squamates by the presence of gastralia (abdominal ribs), enclosed temporal fossae (depressions) in the skull, and the unique replacement of premaxillary teeth by a beaklike extension of the premaxillary bones. This latter characteristic is also seen in one of the the earliest known rhynchocephalids, Brachyrhinodon taylori from a Late Triassic deposit in Virginia.

The rhynchocephalids were never very diverse. However, one small group of aquatic species, the pleurosaurs, radiated into a small number of genera and species between the Early Jurassic and Early Cretaceous periods (approximately 200–100 million years ago). The pleurosaurs had an elongate body and tail and a streamlined head that suggest an active fish-eating lifestyle. After the Jurassic the presence of rhynchocephalids in the fossil record declined, and as yet none have been reported from the Cenozoic Era.

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